Part of an ongoing series of 5e Rules notes.

Our party’s Druid (it’s also available for Artificer) had this spell and used it pretty constantly from the time it arrived to the time the campaign ended at Level 13.

The damage from Thorn Whip is okay, maybe a bit better in the early days when damage is hard to come by, though it scales nicely (something 5e has done well with cantrips). But its true utility comes with its ability to shove people around the battlefield.

The Spell

Here is the spell description:

You create a long, vine-like whip covered in thorns that lashes out at your command toward a creature in range [30 ft]. Make a melee spell attack against the target. If the attack hits, the creature takes 1d6 piercing damage, and if the creature is Large or smaller, you pull the creature up to 10 feet closer to you.

This spell’s damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).

That’s actually pretty cool. A 30-foot range magical attack (requiring an actual attack roll) that does decent damage and lets you yoink people around the game map (at least closer across the game map) by up to 10 feet.

And it’s a cantrip, so you can be playing with this every single round, if you are so inclined.

Note also that, as a (30-foot reach) melee attack (not a ranged attack), the caster takes no Disadvantage using it while standing next to an opponent. The caster is still at Disadvantage vs prone targets over 5 feet away (the rules don’t differentiate between melee and ranged attacks there). Cover effects also still apply.

Finally, in visualizing this spell, most people imagine the caster holding the whip and swing it themselves. However, there’s nothing in the spell that actually says that — it could be floating in mid-air, erupting from the ground — whatever, and because it’s a spell attack, not a weapon attack — you don’t dexterously swing it, but “command it to lash out.”  It’s magic!

Moving the target around

Those words “pull the creature up to 10 feet” are important, because they make it clear that the caster has a choice about whether to move the target at all or how much. It can be left just as a 1d6 damage attack, with the target still standing where they were, or they can be moved 5 feet or 10 feet (or whatever increments your battle grid has, within that 10 foot limit).

But what does closer mean here? Because of the limited distance being moved, I would (in lieu of a more informed reading) argue that each square needs to be toward the caster, reducing the overall distance each step.

 x  x  x  x  x
 x  x  T  x  x
 x  5  5  5  x
10 10 10 10 10
 -  -  -  -  -
 -  -  C  -  -

So, in the case above, the (C)aster could move the (T)arget into each of the numbered points at 5 feet; if moving 10 feet, they would have to got to one of the 10 foot marks. They could not shift into a different 5 foot mark, and definitely not into any of the (x) squares because the move to those is further or the same distance from the Caster.

(Note: Some of this may depend what rule you are using to judge distance on a grid.) (Also Note: A little flexibility here from the DM can fulfill the Rule of Cool.)

Kind of a drag

A lot of questions are raised by the pulling aspect of Thorn Whip (is the victim dragged? catapulted? floated through the air? teleported? and why is there no Strength Save?), but a main use for this power is dragging someone into a hazard — off a cliff, into a Bonfire spell, into a Moonbeam spell, into a Spike Growth spell, up to the immobilized Barbarian, etc.

Is this legit? And (when) does the victim take damage from those hazard areas? The answers are, “Yes” and “It depends.”

Let’s start off by noting that Opportunity Attacks will not be triggered by being yoinked away by a Thorn Whip. That’s pretty much straight out of the book:

You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your Movement, Action, or Reaction. For example, you don’t provoke an opportunity attack if an explosion hurls you out of a foe’s reach or if gravity causes you to fall past an enemy.

Since being yoinked by a Thorn Whip doesn’t use your Movement, Action, or Reaction, no OA is triggered.

That said, it is considered completely legit to involuntarily move someone into a hazard (p. 19) through a spell or force like Thorn Whip:

Entering such an area of effect needn’t be voluntary, unless a spell says otherwise. You can, therefore, hurl a creature into the area with a spell like Thunderwave. We consider that clever play, not an imbalance, so hurl away!

The subject in that ruling is on spells creating …

… an area of effect that does something when a creature enters that area for the first time on a turn or when a creature starts its turn in that area.

That includes things like  Blade Barrier, Cloudkill, Spirit Guardians, and Moonbeam. While “creating an area of effect on the creature or moving it onto the creature doesn’t count,” involuntarily entering the area does.

One caveat there:

Keep in mind, however, that a creature is subjected to such an area of effect only the first time it enters the area on a turn. You can’t move a creature in and out of it to damage it over and over again on the same turn.

(Remember that a round in 5e consists of a sequence of each combatant taking their turn. While a round is about 6 seconds, a turn is some (overlapping) slice of that period, ordered by initiative, but not a defined period of time.)

So given a Moonbeam occupying four squares, you could not force an attack from the spell for each square you used Thorn Whip to drag the target through (i.e., if you dragged them through two squares of it, the 5 foot and 10 foot marks of the spell), just for the initial entry square on your turn.

An exception here (of course there is an exception) is something like Spike Growth. Unlike spells like Moonbeam that trigger “when a creature enters the spell’s area for the first time on a turn,” Spike Growth states:

When a creature moves into or within the area, it takes 2d4 piercing damage for every 5 feet it travels.

Within and every 5 feet it travels are the keys here. You can Thorn Whip someone through two squares (10 feet) of Spike Growth and it will take the 2d4 piercing for each of those squares.

Thorn Whip: It’s Magic!

The magical nature of the pulling done by Thorn Whip is interesting. As described:

If the creature is Large or smaller, you pull the creature up to 10 feet closer to you.

So, note first, this targets creatures. You cannot Thorn Whip over to you the idol sitting on the pedestal over there, or Thorn Whip away the sword in someone’s hand (or that they dropped on the floor).

Second, within the parameter of “Large or smaller,” the target gets no choice or control in the matter of being moved. Standing there slack-jawed or holding onto a support beam for dear life with a STRength of 20, the creature doesn’t even get a Save — they just come. It’s magic!

How does the targeted creature actually move? Fly through the air? Dragged along the ground? It’s not just a teleport because they can take damage from environmental and magical conditions each step of the way. But the spell also doesn’t tie into movement or movement obstacles — it stays nothing about being “slowed” by Difficult Terrain, for example.

I dunno. It’s magic!

Can you Thorn Whip someone through another creature’s square? If you have defeated the cover that other creature is providing, then the answer would seem to be yes, even if it’s an enemy of the target; the only things the rules don’t permit is leaving them in another creature’s square unless it fits other movement/size rules.

What about other obstacles? Assuming you can see past/around them, can you pull a Thorn Whipped person through an obstacle they couldn’t move through themselves? I’d say not, as a general rule; they’ll have to be pulled around.  (But hold this thought for a moment …)

Showing Restraint vs Thorn Whip

What if the target is restrained in some way — grappled, or Entangled, or held by Black Tentacles, or even shackled to a wall? Can Thorn Whip just pull them over regardless? Remember, the individual creature is powerless to stop themselves from being pulled by the spell. But can outside forces prevent it?

Boy, can you find a lot of online argument about that!

General conclusions I’ve drawn on these questions:

• Thorn Whip breaks a grapple, because the grapple rules literally allow for the grapple to be broken by some outside force.
• Against spells that Restrain, like Entangle or Black Tentacles, two alternatives are suggested and, to be honest, I vacillate between them as I reductio ad absurdem each case:
  • Being Restrained does not prevent Forced Movement unless the cause says so (maybe the Entangling vines aren’t deeply rooted). So Thorn Whip will pull an Entangled character from the point where they were Restrained (and ignoring the Difficult terrain), but they will remain Restrained where they land.
  • Being Restrained should resist against Forced Movement. In which case, turn it into a spell contest by the Thorn Whip caster versus the Entangled caster — e.g., the Thorn Whip exerts a “Strength” for the Entangle spell’s Strength Save of d20 + proficiency + stat bonus.  This makes logical sense, but (a) isn’t RAW, and, more importantly (b) starts implying that strength of some sort can counter Thorn Whip, and it’s clear (from lack of a Save against being yoinked) that’s simply not true.
• Against actual physical restraints (being shackled to the wall) … well, it works like the spells mentioned above:  either Thorn Whip just moves the target creature regardless of the shackles (because it’s magic!), or make the Thorn Whip save with the spell strength vs a DC 20 for the manacles.
  • In either case, no additional damage should be done to the target. It’s only a freaking cantrip, fergoshsakes.

This escalating conflict between the Thorn Whip‘s clear it’s magic! nature, which is baked into the language the spell, and the voice of reason as restraints become bigger and more powerful, can only end in things like “I try to Thorn Whip the target through the bars of the jail cell,” and what silliness that results in. At some point the DM has to step in and adjudicate something that feels right while fitting the Rule of Cool.

One final  weird factor in all of this is that the duration for Thorn Whip is “instantaneous.”

Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can’t be dispelled, because its magic exists only for an instant.

That is, it’s not faster than the eye (you can see the whip, you can see it strike, you can see the yank, you could theoretically Counterspell it), but it happens faster than can be addressed or exploited by, for example, a Dispel Magic (or cutting the whip with your sword, or using the whip to make a gibbet, etc.).

Bearing in mind that D&D is not a tool for modeling physics, Thorn Whip is a spell whose nature and execution does not bear too close an examination. Take it as written. It’s magic!

Is Thorn Whip a magical weapon or not?

I keep saying “it’s magic,” but when does it count as magic? This question can come up in a number of circumstances — in my game, it was when the Druid used Thorn Whip on a Gargoyle, which is “resistant to bludgeoning, piercing, and slashing from nonmagical weapons.” Does Thorn Whip qualify, or not? Is Thorn Whip a magical weapon?

As one commenter summarized the argument:

• YES: It’s created by a spell, it uses a melee spell attack to hit, and the spell damage increases with level.

• NO: The spell description only mentions piercing damage, from an object created by the spell, not from the spell directly.

Arguments for Yes, it’s a magic attack

• Because it’s a melee spell attack roll, not a normal melee weapon attack roll, the resistance to weapons doesn’t apply. Melee spell attacks follow the same rules as melee attacks; in this case, a melee attack with a 30 foot range. But it uses the spell attack modifier (spellcasting ability + proficiency) to hit, so, again, it’s a spell attack and ignores the resistance. 
• The Sage Advice Compendium notes (p. 21), in determining if something is magical, qualifying questions would include “Is it a spell? … Is it a spell attack?” This is a spell, and uses a melee spell attack.
•  Mike Mearls (one of the 5e designers) agrees that “any piercing, bludgeoning or slashing damage from spells count as magical in nature.”
• The Monster Manual notes “Particular creatures are even resistant or immune to damage from non-magical attacks (a magical attack is an attack delivered by a spell, a magic item, or another magical source).” This attack is delivered by a spell.
• The whip both magically appears and disappears. That indicates it’s not some sort of physical item being created, but a magic construct.
• The whip not only does damage, it magically lets you pull something closer to you without any additional roll (or save). Thus the overall attack is magical.

Arguments for No, it’s a non-magical weapon attack

• The name of it is a weapon. And the spell actually creates a whip, which is a weapon. So it’s a weapon, crafted by non-conventional means.
• The spell itself doesn’t do the damage; the whip created by it does. Again, the spell doesn’t indicate it creates a magical whip, just a long, vine-like whip that the spell allows you to commend.
• And it does piercing  damage, like a weapon, not magical damage (force, radiance, necrotic, etc.).
• That the damage increases with level doesn’t mean it’s additional magical damage, but could be additionally pointy / strong non-magical thorns.

Conclusion

Net-net, I am persuaded that Thorn Whip is a magical / spell attack (i.e., textualist arguments aside, the vine-like whip is an embodied spell, following the arguments around Spiritual Weapon), so it would defeat non-magical weapon resistance or immunity.

Of course, as an extension of that, something like an Antimagic Field would affect the vine reaching a target within it (even if the caster was outside of the field). It could also be countered, as noted, by a Counterspell.

Because … it’s magic!